Inorganic solids of colloidal size, less than 0.1 micrometer in diameter, are produced for a variety of commercial applications. Among these are: as pigment extenders and for rheological control in coatings; as fillers and reinforcers of elastomers and plastics; as catalysts and catalyst supports; for textile finishing; and as additives for preparation of glasses and ceramics. These products consist of particles which are of irregular spherical shape which readily agglomerate or aggregate to form large clumps that are difficult to redisperse in the appropriate end-use system. Thus, full advantage of their colloidal properties cannot be realized. Many are limited to low temperature applications because of their unstable composition or the sensitivity of their surface properties to high temperatures.
Colloidal particles in acicular form, however, offer advantages in their lesser tendency to aggregate and agglomerate and are therefore more useful because of their greater ease of dispersion to their ultimate particles. In addition, rheological control of colloidal dispersions is more easily effected by relatively smaller additions of acicular particles and, when the acicular particles are used as a filler, the product produced is significantly improved in strength properties due to the improved reinforcing provided by the acicular particles. Use as catalysts or catalyst supports is enhanced because of looser packing of acicular particles and applications as filters and insulating products are possible. The useful range of applications can be extended if the product consists of a phase stable at high temperatures.
The processes developed thus far to produce acicular particles of phases that are stable at high temperatures give products larger than colloidal size for application as insulation materials and for reinforcement of plastics, ceramics and metals, and the conditions required are generally too exacting for commercial application. Examples of these are the spinnable mullite fibers of Berry in U.S. Pat. No. 3,104,943, aluminum silicate fibers of Johnson, et al. in U.S. Pat. No. 3,321,271, Wainer in U.S. Pat. No. 3,023,115 and Jacobson in U.S. Pat. No. 3,607,025.